Generally, in an atomic power plant, steam having finished its task in a high pressure turbine contains approximately 12% hygroscopic moisture. The hygroscopic moisture is typically present in a state of water drops contained in the steam or water adhered to wall surfaces of the apparatus and piping.
When the hydroscopic moisture content in the steam is considerably increased, the hygroscopic moisture may tend to frequently collide with wall surfaces of apparatuses, such as a turbine blade installed in the turbine, thus causing erosion that may leads to serious damage of the equipment. In a low pressure turbine, the turbine efficiency becomes higher as the humidity of the steam fed to the low pressure turbine is greater.
To solve this problem, a hygroscopic moisture separating and heating apparatus for separating the hygroscopic moisture from steam and heating the steam has been provided between the high pressure turbine and the low pressure turbine, in order to remove the hygroscopic moisture from the steam fed from the high pressure turbine, heat the removed steam, and feed the so-heated higher temperature steam into the low pressure turbine. As the hygroscopic moisture separating and heating apparatus of this type, examples of the construction are described in, for example, JP2-242001A and JP9-329302A.
A conventional hygroscopic moisture separating and heating apparatus is now described with reference to FIGS. 9 and 10. FIG. 9 is a lateral cross section showing a construction of the conventional hygroscopic moisture separating and heating apparatus 70, and FIG. 10 is a longitudinal cross section of the hygroscopic moisture separating and heating apparatus taken along line E-E of FIG. 9.
As shown in FIGS. 9 and 10, the hygroscopic moisture separating and heating apparatus 70 includes a main body container 51 having a lateral and cylindrical shape, a hygroscopic moisture separator 58 incorporated in the main body container 51 and adapted to remove hygroscopic moisture from steam 85 to be heated, and U-shaped pipes 52 each located above the hygroscopic moisture separator 58 and adapted to heat the steam 85 to be heated.
In such a construction, the hygroscopic moisture separating and heating apparatus 70 is located to be symmetrical about an imaginary central plane F-F defined at the longitudinal center of the main body container 51.
Through each U-shaped pipe 52, heated steam 86 for heating the steam 85 to be heated is fed. As the heated steam 86, extraction steam supplied from the high pressure turbine of the atomic power plant or main steam supplied from a nuclear reactor can be mentioned.
Each U-shaped pipe 52 includes an advancing part 521, a retracting part 522 located below the advancing part 521, and a U-shaped part 523 connecting the advancing part 521 with the retracting part 522. Each U-shaped pipe 52 is attached to a header 53 so as to constitute each pipe bundle 54, wherein the header 53 is located outside the main body container 51 and adapted to supply and discharge the heated steam 86.
Below the main body container 51, for example, three hygroscopic moisture separators 58 are located along the longitudinal direction.
Steam inlets 56, though which the steam 85 to be heated is fed into the main body container 51, are provided at a bottom portion of the container 51, and steam outlets 57, through which the steam 85 to be heated is discharged from the main body container 51, are provided at a top portion of the container 51.
The steam 85 to be heated, which was fed from the high pressure turbine and supplied into the main body container 51 via each steam inlet 56, passes through each hygroscopic moisture separator 58, so that the hygroscopic moisture can be removed from the steam 85. Thereafter, the steam 85 to be heated is flowed in the main body container 51 upward orthogonally to the advancing part 521 and the retracting part 522 of the U-shaped pipe 52. Consequently, the steam 85 to be heated can be heated due to the heating steam 86 flowed through the U-shape pipe 52, and is then discharged from the main body container 51 via each steam outlet 57. Chain line arrows, as depicted in FIG. 10, for expressing the steam 85 to be heated, respectively designate directions in which the steam 85 to be heated is flowed in the main body container 51.
In the conventional hygroscopic moisture separating and heating apparatus 70, however, the temperature of the steam 85 to be heated becomes higher as it is flowed upward. Therefore, a temperature difference should occur between the advancing part 521 and the retracting part 522 in the U-shaped pipe 52, thus causing a significantly great difference in the amount of condensation of the heated steam 86 in the U-shaped pipe 52 due to cooling.
Namely, in the retracting part 522 of the U-shaped pipe 52, the temperature of the steam 85 to be heated in contact therewith is still relatively low. Therefore, the heated steam 86 flowed in the retracting part 522 may be unduly cooled, thus causing excessively rapid condensation. On the other hand, in the advancing part 521 of the U-shaped pipe 52, the temperature of the steam 85 to be heated in contact therewith is higher, as compared with the case of the aforementioned lower retracting part 522. Thus, the degree of being cooled for the heated steam 86 flowed in the advancing part 521 is significantly lower, as such a greater amount of the steam remains uncondensed.
In such a state, the flow rate distribution of the heated steam 86 in the U-shaped pipe 52 is likely to be unstable, and a periodic temperature change may tend to occur in the U-shaped pipe 52, leading to damage of the U-shaped pipe 52 due to thermal fatigue.
To avoid this problem, a method has been employed, in which a venting pipe (not shown) for venting non-condensable steam of the heated steam 86 is connected with an inlet portion of the U-shaped pipe 52, such that about 5% of the total amount of the heated steam 86 prior to being fed into the venting pipe can be directed into the venting pipe.
However, if the amount (or venting flow rate) of the heated steam 86 to be fed into the venting pipe is considerably large, the amount of the heated steam 86 fed into the U-shaped pipe 52 is of course reduced, thus degrading the thermal efficiency of the entire hygroscopic moisture separating and heating apparatus 70. Therefore, there is a need for reducing the venting flow rate.